Due to integration of CMOS logic process and image sensor process, the image sensors of today are capable of producing complicated analog circuits or digital circuits and signal processing units on a sensor chip. One of the significant applications is an image sensor equipped with an analog-to-digital converter (A/D converter) on an image sensor chip wherein pixels are arranged two-dimensionally.
A column parallel A/D conversion architecture having respective A/D converters for each column is particularly used to mount A/D converters onto an image sensor. Since this method enables the conversion rate per A/D converter to be lowered from a per-pixel reading rate to a per-row reading rate, the speed of the A/D converter itself can be reduced, in turn reducing total power consumption, and as a result speeding-up of the reading rate of the image sensor can be achieved more easily.
The aforementioned image sensor using column parallel A/D conversion conventionally included a lamp-type image sensor which sweeps triangular waves, as disclosed in Japanese Patent Application Laid-Open No. H05-048460, a successive approximation type image sensor, as disclosed in U.S. Pat. No. 5,880,691, and an image sensor using a method wherein a reference voltage is discharged at a rate determined by a pixel's output voltage, as disclosed in Japanese Patent Application Laid-Open No. 2002-033962.
Successive approximation type image sensors have limited application, since large circuits are required to ensure accuracy, and therefore necessitate larger image sensor chip sizes. On the other hand, lamp-type image sensors and image sensors using a reference voltage discharging-type A/D converter are superior in that they allow a more compact-sized circuit.
An example of an image sensor with a lamp-type A/D converter, as disclosed in Japanese Patent Application Laid-Open No. H05-048460, is shown in FIG. 22. In each row of the lamp-type A/D converter is a digital memory comprising a voltage comparator 10, a switch 11 and a digital data accumulation portion 12, and every digital memory is connected to a common counter 5. A signal from a pixel is input as an analog signal via a transfer switch 3 to one end of the voltage comparator 10, while a triangular wave from D/A converter 9 is applied to the other end to have the digital memories of each column retain the value of the counter when the comparator of each column is inverted. Since the triangular wave changes its voltage as it synchronizes with the counter 5, for instance a 8-bit A/D converter requires a processing time of 2 to the 8th power, or 256, steps for sweeping the triangular waves.
An example of an image sensor with a reference voltage discharge-type A/D converter, disclosed in Japanese Patent Application Laid-Open No. 2002-033962, is shown in FIG. 23. While it comprises a voltage comparator and digital memory in the same manner as a lamp-type A/D converter, it first accumulates a constant reference voltage as an electric charge at the comparator, and discharges the charge in the form of an electric current proportional to a pixel signal that has been voltage/current converted at a current mirror circuit 2315, and finally counts the time until the comparator inverts.
FIG. 24 shows an example of a successive approximation type image sensor, as disclosed in U.S. Pat. No. 5,880,691. The successive approximation-type comprises in each row a reference voltage generator which uses a voltage comparator, a digital memory and a digital-to-analog (D/A) converter. A signal from a pixel is applied to one end of the voltage comparator, while a voltage from the reference voltage generator is applied to the other end. Based on the comparison results of the comparator, the reference voltage generator successively changes value, and, for instance, an 8-bit A/D conversion requires a processing time for 8 steps.
The conventional methods described above successively input various types of reference voltage from the reference voltage generator to the comparator, and count the time until a match with the pixel signal is found, instead of comparing a reference voltage and a pixel signal using discharge/charge times. Therefore, since it is necessary to generate multiple reference voltages without variation, production yield will be lowered due to greater circuit size.
As described above, in image sensors with A/D converters built-in in a column parallel pattern, problems have arisen when attempting improvements in speed and accuracy of the A/D converters while maintaining circuit size. For the A/D converter built-in image sensors exemplified above, the reasons for the difficulty in improvements in speed and accuracy while maintaining circuit size will be explained below.
Firstly, in image sensors using lamp-type A/Ds, there is a problem of a slower conversion rate when the number of bits is increased. In lamp-type A/D converters, conversion of N bits requires 2 to the Nth power comparison steps. For instance, when N=12, a great number, such as 4096, of steps are required.
Secondly, in image sensors using lamp-type A/Ds, there is a problem of difficulty in improving speed for further multi-bit enhancement, due to the difficulty of shortening unit-time per step. Since a triangular wave is supplied to the entire sensor face as an analog voltage, it is impossible on principle to shorten the duration of each step beyond a certain duration determined by a RC time constant, required to stabilize the triangular wave output throughout the entire chip. Therefore, when the number of steps is increased due to further multi-bit enhancement, it is impossible to achieve faster speed by shortening duration of each step.
Thirdly, in image sensors having reference voltage discharge A/D converters, the current value will be significantly low when the pixel signal is significantly low during discharge of a constant voltage. Therefore, inversion of the comparator due to discharge requires a long waiting period, suggesting that the underlying principle itself is inappropriate when considering improvement of speed. Furthermore, the circuit for converting voltage to current is insufficient, and the charge gradient displays a significantly nonlinear behavior.
Fourthly, in image sensors using successive approximation-type A/Ds, increase of chip dimensions become a problem. For successive approximation-type A/D converters, the underlying principle places great emphasis on the precision of D/A converters for generating reference voltage. In order to maintain precision, it is necessary to design a circuit while providing enough allowance to effectively ignore the influences of variations resulting from the production process, and as a result, the resistance and capacitance for generating a reference voltage becomes significantly large among the chip, lending to an increase in chip dimensions.
As seen in the problems described above, no solutions currently exist to bring into realization an image sensor equipped with a high-precision, high-speed column-parallel A/D converter with a compact circuit size suitable for image sensors.